Check if you have any pending uploads in OneDrive. Or check if you've recently opened a really large file from your OneDrive, or many larger files in the same timespan. Like a video or something. OneDrive caches that too for you to view.
Those are the only two possibilities as far as I know.
There used to be a really weird bug with pending uploads in OneDrive where I had the same weird thing, making OneDrive unusuable. But one of the updates sort of seemed to fix that (well, it doesn't upload still, but I can remove pending uploads now).
To fix the first: cancel all pending uploads and downloads. To fix the 2nd... I don't really know. One suggestion is to find the large file and rename it, then open the OneDrive app on your phone again, see if that helps. Make it clear it's cache that way.